In recent years, there is an ever-increasing tendency towards high-density integration, high function, and high-density assembly of semiconductors extensively used, for example, in electronic equipments, communication instruments, and personal computers. This has led to a demand for laminated sheets that possess excellent properties in terms of heat resistance and low water absorption and are used in metal foil-clad laminated sheets for printed wiring boards. Laminated sheets of FR-4 type in which the epoxy resin is cured with dicyandiamide have hitherto been extensively used as the laminated sheet for printed wiring boards. This type of laminated sheets could have not satisfactorily met the requirement for high heat resistance.
Cyanate ester resins are known as resins for printed wiring boards that possess excellent heat resistance. In recent years, for example, resin compositions obtained by mixing bisphenol A cyanate ester resins with other heat curable resins or thermoplastic resins have been extensively used as resin compositions for highly functional printed wiring boards, for example, for semiconductor plastic packages. The bisphenol A cyanate ester resins possess excellent properties in terms of electrical properties, mechanical properties, chemical resistance, and adhesion, but on the other hand, sometimes exhibit unsatisfactory water absorption and hygroscopic heat resistance under severe conditions. Accordingly, cyanate ester resins having other structures are under development with a view to further improving properties.
With the tendency towards high-density integration, high function, and high-density assembly of semiconductors, an importance has been placed on a high level of heat dissipation in printed wiring boards. For this reason, laminated sheets having high heat resistance, low water absorption, and high heat conductivity have been desired. Resin compositions having high heat resistance and low water absorption have been proposed as resin compositions for such laminated sheets (patent document 1). The resin compositions, however, had unsatisfactory heat conductivity.
Resin compositions containing inorganic fillers added for heat conductivity improvement purposes have also been proposed (patent document 2). The addition of inorganic fillers, however, has led to an enhancement in water absorption of the resin compositions, making it difficult to simultaneously meet high heat conductivity and low water absorption requirements.